Movable catalyst bed reactors are well known for the catalytic desulfurization and demetallization of petroleum feedstocks. Such reactors will often comprise a normally vertically extending, substantially cylindrical vessel having separate inlet means for reactant and catalyst at the upper part of the vessel, and containing catalyst bed supporting and guiding means in the shape of one or more downwardly converging conical surfaces, with one or more vertical outlet channels connected to the lower end(s) of the conical surface(s), the supporting and guiding means comprising one or more screen sections for withdrawal of reactor effluent from the catalyst, the vessel being further provided with separate outlet means for reactor effluent and catalyst in a lower part of the vessel. Movable catalyst bed reactors of a design as identified above are described e.g. in British Pat. Nos. 8,024,870 and 8,120,176, and in U.S. Pat. No. 3,883,312.
In movable catalyst bed reactors normally catalyst particles are employed having diameters in the range of between about 0.1 and 10 mm, and in particular 1-3 mm. Preferably the catalyst particles are of spherical shape, but cylindrical particles, e.g. extrudates, may be used too.
The use of movable catalyst bed reactors appears to be quite satisfactory from a chemical point of view, but from a mechanical point of view some improvements could still be made to reduce the risk of malfunctioning. One of the problems frequently encountered is plugging of the separating screens (which should retain the catalyst particles while letting the fluid reaction products pass through) by chips and fines. Also, some fouling and erosion of the fluid conduits, pumps and valves forming part of the reactor effluent withdrawal system of movable catalyst bed reactors have been experienced.
An investigation into these problems has revealed that in movable catalyst bed reactors high solid stresses may occur, depending on reactor geometry and pressure drop, thereby causing breakage of the catalyst particles and formation of irregularly shaped chips and fines. It appears in particular, that some particles, which at first sight seem to be spherical actually contain concealed cavities, which render them likely to collapse under pressure. The smallest chips and fines can pass through the separating screens and flow with the reactor effluent to various conduits, pumps etc. which is highly undesired. The larger chips do not pass through the separating screens, but they unfortunately get stuck in the screens' openings, thereby obstructing the passage of reactor effluent. It will be clear that spherical and almost spherical particles are less likely to plug the reactor separating screens since they are able to roll over the separating screens, provided their size exceeds that of the screens' openings.
One solution for reducing the problem of screen plugging is described in Applicant's copending British patent application No. 8133526. According to this publication the catalyst particles are carefully picked out before supplying them to a movable catalyst bed reactor, in order to provide an easily flowing catalyst mass possessing a high intrinsic crushing strength. This solution is independent from the reactor geometry and the operating conditions in the reactor itself. It has now been found that the risk of plugging of the screens' openings in a movable catalyst bed reactor can be further reduced and even substantially eliminated by properly shielding the separating screens against the inflow of irregularly shaped catalyst particles.